In one month exactly, it will be Penny's eighth birthday. She'll be allowed to ride in a car without using a booster seat. So when we leave for Cancun the next morning, she'll be sitting a few inches lower than usual.
The timing works out pretty well -- we'll take her booster seats out of our cars and put them on a shelf for a few months until Alex turns four and is big enough to ride on a booster instead of in a toddler's seat.
And then shortly before Alex, in turn, is big enough and old enough to give up the booster, Penny will turn twelve and will be allowed to ride in the front seat of the car.
And when Alex finally earns that privilege, Penny will have just turned sixteen and will be learning to drive. (Is there anything in the universe that fills me with more terror than the idea of Penny and Alex driving around town on their own? Probably, but I can't think of anything just now.)
It's like a game of leapfrog in four-year increments.